Chapter  put on display the spectroscopic analysis of a miniflare and the associated jet with the IRIS instrument with its high spatial and temporal resolution, which brings exceptional plasma diagnostics of
solar chromospheric and coronal activity during magnetic reconnection ([1]}).
A spatio-temporal analysis of IRIS spectra observed in the spectral ranges of Mg II, C II and Si IV ions are proceed and the Doppler velocities from Mg II lines are computed by using a cloud model technique. Strong asymmetric Mg II and C II line profiles with extended blue wings observed at the reconnection site are interpreted by the presence of two chromospheric temperature clouds, one explosive cloud with blueshifts at 290 km s\(^{-1}\)  and one cloud with smaller Dopplershift (around 36 km s\(^{-1}\) ). We used the cloud model technique as a simple diagnostics tool for velocity calculations. Such models are used to derive true velocities which usually differ from those obtained from Doppler shifts, and this is the basic idea behind the cloud
model. Simultaneously at the same location (jet base), strong emission of several transition region lines (O IV and Si IV) and the emission of the Mg II triplet lines of the Balmer-continuum are observed.
Emission at the minimum temperature was detected with AIA 1600 Å and 1700 Å filters and confirmed the low level heating.
Mg II and C II lines are good diagnostics for detecting plasma at chromospheric temperature (T \(<\)  20,000 K). We identified the bilateral outflows in the bald patches, and the bald patch current sheet was transformed to an `X'-point current sheet during the reconnection. The absorption of identified chromospheric lines in Si IV broad profiles have observed and analysed. In fact
Si IV profiles are striped by absorption lines formed at photospheric temperatures and clearly show the presence of cool plasma over transition region temperature material in the reconnection site. This cool plasma is certainly due to the cool clouds, either the ejected fast cloud feed by trapped material in bald patch or by surge plasma.
All this event is finally embedded in the corona.
This demonstrated the possibility of having successive layers in the atmosphere with different velocities and temperatures in the current sheet region. With such observations of IRIS line and continuum emission, we proposed a stratification model for the white-light mini flare atmosphere with multiple layers of different temperatures along the line of sight, in a reconnection current sheet. It was an important result and for the first time that one could quantify the fast speed (possibly Alfvénic flows) of cool clouds ejected transverse to the jet direction by using the cloud model technique. It is concluded that the plasma of ejected clouds could come from the trapped region, where the cool material was stucked before reconnection between the two emerging flux regions or be caused by a chromospheric-temperature (cool) upflow during the magnetic reconnection.
